1. Field of the Invention
This invention relates to signal processing, and, more particularly, to signal processing in diversity transmission systems.
2. Description of the Prior Art
Digital communication over radio transmission networks is susceptible to errors. These errors are caused by signal fading due to atmosphereic conditions and due to signal attenuation caused by physical structures in urban environments, and by electromagnetic interferences from other transmitting systems. Signal transmission over a mobile radio channel is particularly subject to errors due to fading. These errors manifest themselves in clusters or bursts of errors.
The various methods which have been devised to reduce the effect of burst errors generally apply one of two basic approaches: protection of each bit or each word of transmission; or protection of the message as a whole. In accordance with the former approach, error detection and/or error correction codes are generally used. In accordance with the latter approach, it has been found advantageous to use space diversity or time diversity transmission. In space diversity transmission, a transmitted signal is received by a plurality of receiving systems, and the message signal is derived by manipulation of the received signals. In time diversity transmission, the signal is transmitted a number of times by a single transmitter, and the message signal is, similarly, derived by manipulation of the received signals.
For example, Kalliomaki et al, U.S. Pat. No. 3,670,275, issued June 13, 1972, describes a diversity system which measures the broadband signal-to-noise ratio of the signals received from diverse antennae and, based on the signal-to-noise ratio measurements, selects the signal of a particular antenna.
Another system, described by George L. Beers in U.S. Pat. No. 2,059,081, issued Oct. 27, 1936, has a plurality of antennae and switching means for successively connecting the antennae to the input circuit of the single receiver. Selection of an optimum antenna is based upon a measure of the signal's strength at the antenna's output.
Still another system is described by Oswald et al in U.S. Pat. No. 3,737,783, issued June 5, 1973. Oswald et al disclose a space diversity system comprising two antennae having their signals applied to an adder, a subtractor, and to various cross-correlators which develop output signals that are combined to form a message signal.
The above prior art systems require fairly complex and often expensive circuitry for receiving and developing the desired message signal.